The discovery that glomeruli are normal in size relative to body weight and tubules shorter by 30% than normal in kidneys of hypothyroid rats has proved useful as an approach to study of glomerulotubular imbalance and nephron heterogeneity. Renal growth will be measured in littermates to determine if growth arrest or atrophy is the cause of the reduction in size of the kidney. Since glomerular filtration has been found to be reduced in arterial pressure it may be inferred that dimensional glomerulotubular imbalance is corrected to some extent by an opposing functional imbalance. To assess this possibility, the relationship between glomerular function and tubular transport capacity will be evaluated by glucose titration and by intracortical distribution of single nephron glomerular filtration rates.